1. Field
This patent specification describes a switching regulator, and more particularly, a switching regulator capable of outputting positive and negative voltages.
2. Background Art
Recently, portable devices have become compact and are offering an increasing range of functions. For example, a mobile phone commonly mounts a digital camera as a standard function in addition to a primary function of a communication capability, i.e., transferring and receiving functions. Further, the mobile phone typically includes a variety of electric functional elements, such as a display panel using a liquid crystal display (LCD) and organic electroluminescence (EL), an image pickup device such as a charge-coupled device (CCD), and memories. For such mobile phone, a variety of voltages including positive and negative voltages are required to drive the electric functional elements described above. Further, there is market demand to make the mobile phone more compact and consume less power.
To generate both the positive and the negative voltages required, a variety of converters have been proposed, for example, a DC-DC converter that employs a plurality of inductors and a charge-pump-type converter that employs a plurality of capacitors. Using a plurality of inductors, it is possible to supply current for a large load, however, it is difficult to make the inductors compact. Further, an apparatus may increase in size because of the large room required for the plurality of inductors.
The charge-pump-type converter needs a lot of capacitors. Further, when a load current increases, a size of the capacitor must be increased in proportion to a necessary load current. For this reason, applications for the charge-pump-type converter are limited in the field of compact portable devices.
FIG. 1 illustrates an example of a power supply circuit that generates both the positive and the negative voltages using a single inductor. In FIG. 1, the power supply circuit includes an NMOS transistor M101, a PMOS transistor M102, diodes D101 and D102, an inductor L101, capacitors C121 and C122, resistors R121 and R122, and a timing pulse generator 130. When the PMOS transistor M102 is on to create a conduction state, the inductor L101, the NMOS transistor M101 and the diode D101 form a step-up switching regulator. Therefore, the power supply circuit performs a step-up operation by switching the NMOS transistor M101 on and off, and outputs a positive voltage from a positive voltage output terminal OUTa.
Meanwhile, when the NMOS transistor M101 is on to create a conduction state, the inductor L101, the PMOS transistor M102 and the diode D102 form a polarity-inversion-type switching regulator. The power supply circuit outputs a negative voltage from a negative voltage output terminal OUTb by switching the PMOS transistor M102 on and off.
FIG. 2 is a circuit diagram of the timing pulse generator 130 shown in FIG. 1. The timing pulse generator 130 includes PWM comparators 132 and 134, operational amplifiers 131 and 133, a triangular wave generator 135, resistors R131, R132, R133, R134, R135 and R136, and reference voltage generators P1 and P2. The timing pulse generator 130 generates and outputs pulse signals G1 and G2 which drive the NMOS transistor M101 and the PMOS transistor M102, respectively. The operational amplifier 131 amplifies a voltage difference between a divided voltage divided by the resistors R131 and R132 and the reference voltage P1. The divided voltage divided by the resistors R131 and R132 is generated by dividing a voltage difference between a voltage at the positive voltage output terminal OUTa and a voltage at the negative voltage output terminal OUTb. The PWM comparator 132 generates the pulse signal G1 based on an amplified signal by performing PWM (pulse-width modulation) using a triangular wave generated by the triangular wave generator 135.
Similarly, the operational amplifier 133 amplifies a voltage difference between a divided voltage divided by the resistors R133 and R134 and the reference voltage P2. The divided voltage divided by the resistors R133 and R134 is generated by dividing a voltage difference between the voltage at the positive voltage output terminal OUTa and a voltage at the negative voltage output terminal OUTb. The PWM comparator 134 generates the pulse signal G2 based on an amplified signal by performing PWM using a triangular wave generated by the triangular wave generator 135.
When the pulse signal G1 is high and the pulse signal G2 is low, the NMOS transistor M101 and the PMOS transistor M102 are both on to create a conduction state so that the inductor L101 stores energy. When the pulse signals G1 and G2 are both low, the NMOS transistor M101 is off and the PMOS transistor M102 is on. Therefore, the energy stored in the inductor L101 is stored into the capacitor C121 that is connected between the positive voltage output terminal OUTa and ground, and is also output from the positive voltage output terminal OUTa.
When the pulse signals G1 and G2 are both high, the NMOS transistor M101 is on and the PMOS transistor M102 is off. The energy stored in the inductor L101 is stored into the capacitor C122 connected between the negative voltage output terminal OUTb and ground, and is also output from the negative voltage output terminal OUTb.
However, the power supply circuit shown in FIG. 1 uses the diodes as a rectifier, which causes a large voltage drop. Accordingly, power conversion efficiency is decreased especially when an output voltage of the power supply circuit is small. Further, as for the circuit configuration for outputting a positive voltage, it is not possible to output a small voltage smaller than the input voltage because the power supply circuit is a step-up voltage circuit.